The principal neurons of the medial superior olive (MSO) process interaural time differences (ITDs), cues used for localizing sounds along the horizontal plane as well as detection of speech and communication signals in noise. Despite intense interest in understanding the circuit dynamics that underlie ITD processing, there is surprisingly little experimental data available to drive current models of binaural hearing and MSO function. This is in part due to the fact that the biophysical specializations that are central to MSO neurons< functional role make these neurons unusually difficult to record in vivo with traditional extracellular recording methods. Although the advent of patch-clamp recordings from MSO neurons in brain slices has enabled detailed studies of the synaptic properties of MSO neurons, the obligatory disruption of circuit connections has precluded these studies from accurately reproducing the physiological timing and balance of binaural inputs. The current proposal will provide the first explicit effort to bridge the conceptual and technical gaps between cellular electrophysiological approaches in vitro and whole-animal acoustic investigations in vivo. First, we will employ patch-clamp recordings from MSO neurons in vivo and record responses to both monaural and binaural stimuli. We will use intracellular methods to compare the responses to ipsi- and contralateral acoustic stimulation in terms of their timing, shape, and (a) symmetry. We will test whether these monaural responses allow us to predict the binaural responses of the same neurons, which we will also measure. Second, in order to interpret in vivo experiments, we will develop a slice preparation containing the auditory nerve as well as the entire cochlear nucleus and superior olivary complex. By synchronously stimulating one or both auditory nerves during whole-cell recordings from MSO neurons, we will reproduce in vitro the natural timing of excitatory and inhibitory synaptic inputs, the details of which are the basis for all current models of binaural processing in the MSO. The impact and timing of inhibition will also be explored by comparing control responses to those evoked during pharmacological blockade of glycinergic inhibition. PUBLIC HEALTH RELEVANCE: Binaural cues are critical for speech perception and sound localization, and in recent years have become critical components of stimulation paradigms in auditory prostheses. An understanding of how binaural cues are processed by the brain is thus a prerequisite for the development of effective solutions for deafness and other hearing disorders.